1. Field of the Invention
This invention relates to a metal electrode design for electrolysis cells wherein aqueous solutions of alkali metal chlorides are electrolyzed.
2. Prior Art
In the electrolysis of aqueous solutions of alkali-metal halides in electrolytic cells employing interleaved anodes and cathodes, it has been well known for many years that the environment around the anode is very corrosive to most conductive metals. Hence, for many years anodes made of graphite have been used instead of the metals normally thought of as conductive metals; e.g., copper, aluminum and steel. Yet it is known that even graphite anodes deteriorate, albeit slowly, in the hostile anolyte environment.
This deterioration is known to cause a slow reduction in size of the graphite anode. Such a reduction in size, of course, means a widening of the interelectrode gaps between the several interleaved anodes and cathodes. This gap width increase is well known to cause an increase in the cell operating voltage, and, hence, a very appreciable increase in the electrical power cost required for producing the same amount of product.
The electrical power consumption for such cells is determined by multiplying the cell operating voltage by the cell current; i.e., power=voltage x current. It is known that one of the ways to reduce power consumption is to reduce the cell operating voltage; i.e., reduce the voltage in the power equation given above. Inasmuch as very large amounts of current are required in such electrolysis processes, it is economically very desirable to reduce the cell operating voltage even by only a few millivolts. Several improvements in anode, cathode, and cell design have been invented to accomplish this voltage decrease.
In the more recent past, metal anodes made of special non-corrosive metals having catalytic coatings have been discovered as replacements for the relatively wide graphite anodes. Such special metals include valve metals or film forming metals such as titanium, tantalum, niobium, zirconium, etc., with titanium being preferred due primarily to its lower cost. Typically the catalytic coating is either an oxide of ruthenium or spinels of cobalt.
The anodes made from these metals have often been designed to replace the graphite anodes in pre-existing cell designs wherein the cathodes are interleaved between the anodes. Thus graphite blades of thickness of about 1/2 inch are generally replaced by anodes with each having two parallel sheets of about 1/16 inch thickness serving as metal working faces. These two sheets are spaced far enough apart to be near the adjacent cathodes, the spacing apart of said cathodes generally being set for cells designed for the wider graphite anodes. Examples of cells containing this kind of anode can be found in many references, particularly in references published in the last decade. See U.S. Pat. Nos. 3,674,676; 4,110,191; 4,101,410; 3,963,596; 3,591,483; 3,981,790; 4,008,143; 4,026,785; 4,028,214; 4,028,213; 3,941,676; 3,932,261; 3,980,544; 3,852,179; 3,876,517; 3,902,985; 3,910,827; 3,928,150; 3,968,021; and 4,016,064.
These types of anodes, however, have required parts located or manipulations performed inside the narrow space between the two anode working face sheets in order to physically attach them to the anode base. Assemblying such parts require intricate, time-consuming manipulations which become quite expensive when assemblying many cells containing many anodes.
On the other hand there are references which teach the use of L-shaped metal anodes interleaved between cathode pockets. These anodes are typically a single, solid sheet of metal bent in the shape of an "L", as viewed from above, with the two surfaces of the longer portion of the L-shaped metal sheet serving as the anode's working faces. This longer portion of the L-shape anodes extend between the cathodes of the cell while the shorter portion of the L-shaped anode serves as a flange for electrical and mechanical attachment to the anode's supporting base. For examples of such L-shaped anodes, see U.S. Pat. Nos. 3,755,108; 3,919,059; 3,856,651; 3,677,927; and 3,759,813. British patent 1,125,493 shows an anode of two pieces bolted together to form an L-shaped anode. U.S. Pat. No. 1,303,519 to A. T. Stuart (1919) shows L-shaped anodes for the electrolysis of water to produce hydrogen and oxygen gases.
These single L-shaped anodes all have the advantage of being able to be assembled individually as well as being attached to the anode supporting base without workmen having to devise means and methods to work within tight spaces between dual working faces as is done for those metal anodes having two spaced working faces described above.
It has been found, however, that cells employing these L-shaped anodes operate at a considerably higher voltage than those anodes having two spaced working faces between each pair of adjacent cathodes. It would be advantageous, therefore, to have a single anode which had the low voltage characteristics of the metal anodes employing two spaced working faces and yet have the advantage of easy installation of the single L-shaped anodes. This and other advantages are achieved by the surprisingly simple appearing and simply made anode of the present invention, which herein is also referred to as a "double L anode".